Relay PDT

1 Pole Double Throw, as in a Single Pole Double Throw (SPDT), or Form C contact, also known as a 1 pole changeover contact. Inadvisable way to refer to it though, it allows confusion.

NO = normally open. if both are normally open on a SPDT or 1PDT, where 1 = S for single.

then it takes voltage (120 VAC in this case) to make it go either way. When the voltage stops a spring takes it to the dual off position. Usually an SPDT relay is in one position with the power off and those contacts are normally closed (NC) and the others are normally open (NO).

They make toggle switches that way as well, center off and spring return center off

As already stated, it means a Single Pole Double Throw. The number 1 instead of the word single is commonly used because realys typically can have 3, 4 or more poles, including six or seven, the words of which obviously also start with the letter"S".

There is no particular reason in general to use or not to use it in lieu of a 1PST (Single Pole Single Throw) NO contact relay, assuming of course that it meets the required specs for your application. You just wire the NO contact and ignore the NC.

A 1PST relay should be slightly cheaper than the equivalent 1PDT.

Designers often go up a size in number of contacts and also often use double throw relays where they can get by with single throw, because the cost and size penalties are so small and later modifications or additions to the circuit can be facilitated.